This invention relates to measuring biopotential electrical signals particularly relating to electro-oculography and deals more specifically with apparatus for sensing and recording the ocular standing DC potentials indicative of eye movement across the eye socket.
Electro-oculography relates generally to a method of recording voltage changes due to eye movement, from pairs of electrodes attached to the body surface near the eyes and deals more specifically with the production and study of an electro-oculogram (EOG). The EOG is defined in the present disclosure to be a graphic display or record of the measurement of such voltage changes.
It is thought that the voltage developed across the eye is due to electrical activity from a retinal structure and, in particular, activity in the pigment epithelium. The pigment epithelium is the tissue that lines the back of the retina and plays a very supportive role for the retina and particularly the outer segments of the receptors. The pigment epithelium itself plays no direct role in the transmission of information from the eye to the brain. It has, however, been established that a relationship exists between the pigment epithelium and the receptors, such that, if the pigment epithelium is not functioning properly the receptors are also generally not functioning properly. Vision does not occur if the receptors are not working because only the receptors can receive and transform light into the neural activity which is sent to the brain.
The measurement of the ocular standing DC potential or change in voltage across the eye as the eye moves is based on the proposition that the eyeball is charged and behaves as an electrical dipole. Motion of the eyeball produces a varying voltage on the skin surrounding the eye and the magnitude of the voltage difference produced across the eye has been found to be nearly proportional to the position of the eye. Consequently, the EOG provides an indirect measure of the ocular standing DC potential and may be used to derive a precise, continuous record of eye position and, if desired, its time derivative, eye motion.
Measurements made heretofore have shown that the ocular standing DC potential generally ranges in magnitude from 50 to 3500 microvolts for eye fixation changes between 2.5 degrees to 90 degrees, and which average 35 microvolts per degree of eye rotation. Heretofore used DC voltage sensing and amplification means have not proven to be completely satisfactory for the sensing and recording of such low level DC voltage signals. The ocular standing DC potential is often masked in noise levels approaching several millivolts. Prior high gain DC amplifier-recorder systems generally encounter difficulties in sensing the ocular standing DC potential and its variations in noise levels exceeding 1 microvolt.
Another problem associated with the sensing and recording of the ocular standing DC potential is variations in the sensed DC voltages caused by changes in the electrode to skin surface contact. Such contact changes occur during the course of apparatus set up and patient testing and may be caused by sweating, uncleansed skin surface at the contact point and, improper electrode attachment among others. It is desirable that the influence of such skin surface contact changes be eliminated or compensated for so that any variations in the magnitude of the sensed ocular standing DC potential are attributable to voltage changes produced by eye movement.
Sullivan in U.S. Pat. No. 3,217,706 attempts to overcome some of the problems associated with low level DC voltage signal detection and changes in electrode to skin surface contact by amplitude modulating a carrier with impedance changes which occur between pairs of periorbital electrodes as the eye moves, and then demodulating the carrier to provide the representative EOG electrical potential signal and its variations.
Another drawback associated with some prior systems is that the patient under test and the sensing and recording apparatus have to be kept at the same electrical reference potential for proper apparatus operation. Generally, the patient and apparatus are electrically grounded. The patient is connected to electrical ground via an electrode attached to the skin. Systems of this type are generally very susceptible to electromagnetic interference and electrical noise pick up. These systems may also pose a safety hazard because the patient is at a ground potential and consequently, may experience an electrical shock when touching the surrounding which may be at a different electrical potential.
Another drawback to other previously used systems is the requirement of additional equipment, such as, for example, an oscilloscope, which is used to calibrate and balance the sensing and recording apparatus so that the EOG readings have equal excursions above and below a mid-graph reference line for both the left and right eyes when the left and right eyes move an equal amount.
Yet another drawback found in some other previously used systems is the necessity to continuously rebalance the recording apparatus and reposition the mid-graph reference line during testing of the patient. Repositioning of the reference line is necessary to prevent the recording media, such as, for example, a strip chart recorder pen from drifting off the chart paper during testing.
Still another drawback to other systems is the considerable expense associated with the acquisition of such sensing and recording apparatus. Hence, the simple economics of apparatus cost versus expected frequency of use tend to dictate which practitioners and/or hospitals acquire such apparatus. More often than not, practitioners who sometimes find electro-diagnostic analysis to be clinically useful, cannot justify the expense of acquiring such apparatus simply because of the relative infrequency of such occasions.
Toglia in U.S. Pat. No. 4,155,352 describes an interface unit for use with an electroencephalograph (EEG) machine as one way to make such apparatus less costly by using the recording portion of the EEG machine. Conventionally located electrodes sense the voltage signals produced about the eye. The signals are processed and then directly coupled to an EEG machine recorder inputs to produce the EOG.
Toglia also attempts to overcome the problem of noise contaminated signals by employing frequency modulation techniques to transmit and amplify the sensed signals.
Consequently, there exists a need for apparatus to sense and record biopotential electrical signals that is inexpensive, easy to use, and overcomes the limitations generally associated with other systems.
A general aim of the present invention is to provide an improved apparatus for sensing and recording the biopotential electrical signals indicative of eye movement across the eye socket.
A feature of the present invention is to provide apparatus to sense and record the ocular standing DC potential directly without the signal conversion normally associated with modulation techniques.
Another feature of the present invention is to provide apparatus which automatically compensates for variations in the sensed biopotential electrical signals caused by changes in the electrode to skin surface contact.
Other features and advantages of the present invention will be apparent from the following written description and the drawings forming a part hereof.